Differential pressure switch



Oct. 16, 1956 A. w. GAUBATZ 2,767,275

DIFFERENTIAL PRESSURE SWITCH Filed sept. 5, 1951 United States Patent O ice ZWZY DIFFERENTIAL PRESSURE SWITCH Arthur W. Gaubatz, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application September 5, 1951, Serial No. 245%,162

z Claims. (ci. 20o- 81.4)

My invention relates principally to pressure actuated devices and, more particularly, to a switch or other device `actuated by the differential between two pressures. A feature of the switch lies in the provision of means for actuating the switch manually to test the operation of the electrical portion thereof. A further feature of the invention lies in means for damping the operation of the device and for preventing response to transient pressure surges. While the invention may be employed in various connections, in its presently preferred embodiment the invention is incorporated in an aircraft engine fuel system to provide means by which a change in the relation of two pressures, which indicates -a condition in the fuel system, may be signaled to an engineer or pilot of the aircraft in which the engine is installed.

The principal objects of the invention are to provide an improved device for signaling pressure conditions, to provide an improved differential pressure responsive switch, and to provide means for checking the operation of an electric switch.

The nature of the invention and the manner in which the stated objects, as well as other more or less ancillary objects are realized, will be apparent to those skilled in the art from the succeeding detailed description of the preferred embodiment of the inventionand the accompanying drawings, in which Figure l is a longitudinal sectional View of a differential pressure switch in accordance withv the invention, Figure 2 is a longitudinal section, and Figures 3 and 4 are transverse sections, the planes of Figures 2 to 4 being indicated in Figure l.

Since the Iinvention may be employed with various fluid pressure systems, and the details of the fluid system are immaterial to the invention or to the understanding thereof, the fluid system will not be described in detail. Referring to Figure 2, the fluid system may include, for example, a pump body, filter casing, fuel regulator or the like comprising a casing 11 in which are two fluid chambers 12 and 13. The switch of the invention responds to the difference in pressures between the chambers 12 and 13. The switch assembly comprises a base 14 which may be secured to the casing 11 by studs 16. A rectangular post 17 extending from the base 14 mounts a support 18 on which are fixed two expansible bellows assemblies 19 and 21. As will be apparent, the support 18 is generally drum-shaped but is slotted on opposite sides thereof. The slot 22 in the support receives the end of the post or web 17. Machine screws 23 extending through the block 18 are received in tapped holes in the base portion 17.

Passages 24 and 26 in the casing 11 extend from the chambers 12 and 13 respectively to the surface on which the base 14 is mounted where they connect with drilled passages 27 and 28 through the base. These passages communicate with drilled passages 29 and 31 in the block 18 which extend to the opposite faces thereof.

The bellows assemblies 19 and 21 may be identical, each comprising a corrugated metal bellows 32, one end of which is brazed or otherwise fixed in a huid-tight manner to the block 18 and the free end of which is sealed to a metal disk or head 33. The bellows is guided by an internal sleeve 34 fixed to the head 33 which slides on a pin 36 extending from the block 18. The heads 33 are joined by a yoke comprising two strips 37 and 3S, the ends of which are bent over the face of the disk 33 and brazed or soldered thereto.

As will be apparent, a fluid pressure introduced into either of the bellows through the passages described will tend to extend the bellows. Since the two are yoked together and are of the same dimensions, the movement of the yoke 37, 38 will be proportional to the difference between the two pressures, the restraining force being developed by the inherent elasticity of the bellows.

T wo snap switches 41 and 42 of known commercial type are mounted in the usual manner by screws 43 extending through the bodies of the switches on posts 44 extending outwardly from the base 14. The switches are of the type which are actuated in response to a small extent of reciprocation of an actuating plunger 46 extending from the body of the switch. The plungers 46 are in contact with or adjacent the heads 33 so that upon movement of the bellows indicating the pressure differential for which the apparatus is set one or the other of the switches 41 and 42 is actuated. Thus, for example, if the pressure in the chamber 12 is normally greater than that in the chamber 13, the bellows is normally moved upward as viewed in Figure 1 to depress the plunger 46. If the pressure differential decreases or reverses, the switch plunger' of the switch 42 may return and that of the switch 41 may be actuated. Obviously, the switches may be single or double throw, normally open or normally closed, and one only switch may be employed if desired. The switches 41 and 42 are connected by wiring (not shown) to a standard cable connector S1 mounted on an extension 52 of the body from which the wires may be led through an opening 53 in the end of the body and holes 54 in the web 17.

Since the pressure switch may be provided only for the purpose of indicating malfunctioning of the uid systern, the switches may not be caused to operate over a long period of time. lt is highly desirable, therefore, to provide some means for determining whether the switches and the circuits actuated thereby are operative. To this end, a mechanism, which may be termed a check key, is provided by which the bellows assemblies may be moved manually to check the operation of the electric system. This key comprises a shaft 61 (Fig. 4) which is journaled in a portion 62 of the base 14. An arm 63 is splined to the inner end of the shaft and is retained by snap ring 64. The end of the arm 63 is -disposed in a slot 66 in the yoke 38, preferably with sufficient clearance to permit normal movement of the yoke without striking the arm 63. The outer end of the shaft 61 is provided with a head 67 which may be adapted for engagement by a wrench or screw driver so that the shaft may be oscillated to force the bellows assembly through its range of travel and actuate the switches.

Another valuable feature of the device is the provision for hydraulic damping of the movement of the bellows assemblies to prevent undesirable response to short-period transient pressure fluctuations and stabilize the action of the pressureresponsive device and switches. The sleeves 34 are brazed or otherwise fixed in a fluid-tight manner to the heads 33, and the heads of the pins 36 fit in the sleeve with a small clearance to define an annular orifice between the sleeve and pin.

For the bellows assembly to move in either direction, fluid must be forced from within the sleeve 34 past the head of the pin 36. The clearance may be fixed to provide a suitable degree of damping for the particular requirements. This hydraulic frictional damping strongly resists vibration or oscillation of the bellows assembly and delays the response to pressure differential slightly to prevent response to transient pressure or shock wave conditions.

Because of the annular form of the orifice and the relative movement of the sleeve and pin, the orifice is not subject to clogging.

Further damping, and protection of the bellows assemblies against pressure surgcs, is provided by restrictions in the pressure line leading from the chambers 12 and 13 to the bellows. In the preferred form, this is accomplished by pins 72 and 7.3 in the passages 27 and 28. These pins are of such diameter as to slide freely in the passages and provide a restricted orifice for uid flow between the outer surface of each pin and the wall of the passage. Because of the annular form of the orifice and the reciprocation of the pins in the passages with changes in pressure, these orifices are self-cleaning.

The ends of the pins 72 and 73 are cut off at an acute angle to the axis of the pins so that the pins cannot seat at the end of the passages and block fluid flow. The pins cannot enter the passages 29 and 31 because of the small diameter of these passages, and may be prevented from dropping out of the lower end of passages 27 and 28 by peening the ends of these passages to create a shoulder.

The pins 72 and 73 are slightly shorter than the passages 27 and 28 so that limited reciprocation is possible to dislodge foreign matter which might collect in the passages.

The mechanism may be enclosed by a sheet metal cover 69 retained by machine screws 71 threaded into the base 14.

It will be apparent that the bellows 19 and 21 may be of different diameters so that the response to the two pressures is unequal. The term differential is used to refer to a differential between functions Iof pressure such as will be arrived at in case the bellows are of different area. It will also be apparent that the location of the switches may be such that the neutral position of the bellows is that due to a value of pressure differential other than Zero.

- The detailed description of the preferred embodiment of the invention for purposes of disclosing the principles thereof is not to be considered as limiting the invention, since many modifications thereof may be made within the scope of the invention by the exercise of skill in the art.

Y1 claim:

1. VA differential pressure responsive device comprising in combination, a support, two extensible bellows mounted on opposite faces of the support, a yoke coupling the free ends of the bellows for joint movement, the support defining a iiuid pressure connection leading to each bellows, and a switch adjacent the free end of one bellows, so constructed and arranged that the switch responds to a differential of pressure in the two bellows, and manually operable means for reciprocating the yoke to actuate the switch independently of pressure to check the operation of said switch comprising a manually rotatable shaft, an arm thereon, and a lost motion connection between the arm and the yoke.

2. A differential pressure responsive device comprising in combination, a support, two extensible bellows mounted on opposite faces of the support, a yoke coupling the free ends of the bellows for joint movement, the support dening a uid pressure connection leading to each bellows, and an operable device adjacent the free end of one bell-ows, so constructed and arranged that the operable device responds to a differential of pressureV in the two bellows, and manually operable means for reciprocating the yoke to actuate the operable device independently of pressure to check the operation of said operable device comprising a manually rotatable shaft, an arm thereon, and a lost motion connection between the arm and the yoke.

References Cited in the le of this patent UNITED STATES PATENTS 764,572 Fulton July 12, 1904 1,544,314 Hanks June 30, 1925 1,847,086 VGargan Mar. l, 1932 1,948,503 Bijur Feb. 27, 1934 2,049,531 White Aug. 4, 1936 2,147,031 Hastings et al Feb. 14, 1939 2,269,787 Saxe Ian. 13, 1942 2,339,087 Mantz Ian. 11, 1944 2,434,794 Giesler Jan. 20, 1948 2,470,742 Hasse et al May 17, 1949 2,508,584 Steigerwalt May 23, 1950 2,527,171, Allwein Oct. 24, 1950 2,531,479 Southern et al Nov. 28, 1950 2,581,078 Cortield et al Jan. 1, 1952 FOREIGN PATENTS 834,242 France Aug. 8, 1938 

